function ajaxCall(fileName) {
    var ret = null;
    var xmlhttp;
    if (window.XMLHttpRequest) {
        xmlhttp = new XMLHttpRequest();// code for IE7+, Firefox, Chrome, Opera, Safari
    } else {
        xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");// code for IE6, IE5
    }
    xmlhttp.onreadystatechange = function() {
        if (xmlhttp.readyState === 4 && xmlhttp.status === 200) {
            ret = JSON.parse(xmlhttp.responseText);
        }
    };
    xmlhttp.open("GET", fileName, false);
    xmlhttp.send();
    return ret;
}
